allow calls to shell(), with no shell.

easier to build variable argvs with commands
refactor argv building in iaas.ssh()
implement --login, -l or user@host for ssh

when both -l and user@host are provided, mimic ssh behaviour
rename "shell" to execute

just because shell(cmd, shell=True) sounds wrong
(related to dccbe29)
add documentation for the ssh login parameter
Merge pull request #7 from kalou/ssh-opts

Improve vm ssh command to handle login@vm parameter or --login